Organic-halogen compounds, including dichloromethane (CH2Cl2) and chloroform (CHCl3), are widely used solvents in laboratories, industry and hospitals. However, many of such halogenated compounds are suspected carcinogens and are irritants to the eyes, skin and respiratory system. For example, inhaling the vapors of dichloromethane and chloroform can damage the health of operators due to the extremely high volatility and relatively low olfactory-sensitivity of these two solvents. Also, other organic-halogen compounds such as polychlorinated alkanes and biphenyls have become a significant environmental problem because extremely minute levels are believed to present a health risk. Therefore, there is an increasing need to monitor and screen the presence of organic-halogen compounds in the environment. In particular, sensory materials for dichloromethane, chloroform and other volatile organic compounds (VOCs) with high sensitivity and selectivity are in demand for in vivo environmental monitoring and evaluation.
Luminescent probes provide a convenient method for sensor device design and application. The most common photoluminescence sensor utilizes intensity responses to analytes under conditions of constant irradiation. Ideally, the photoluminescence intensity change is selective, reversible, and sensitive to the analyte of interest. Regardless of the origin, intensity-based sensing has the advantage of being straightforward, inexpensive, and easy to implement. Noble metal complexes have been extensively studied as luminescent sensory materials for pH, cations, anions and so on. Recent progress has also shown that some of these complexes may exhibit vapochromic properties and give luminescent responses in the presence of VOCs and gases. These luminescent probes can be divided into three main types in view of the nature of the vapor-complex interaction mechanism: (a) discrete metallocyclophanes possessing large cavities that can accommodate VOC molecules; (b) oxygen-quenching phosphorescent lumophores tailored into mesoporous materials such as zeolite, rubber and sol-gel; (c) molecular solids that change their crystal lattice or chemical structures upon interaction with vapors.
The electronic structures of square-planar platinum(II) complexes are often sensitive to solid-state effects and the polymorphism of diimine Pt(II) complexes is well known. Recently, the development of luminescent Pt(II) complexes has revealed low-energy excited states. Coordinately unsaturated platinum(II) luminophores and their applications as molecular sensors have also generated immense interest. Many investigations have shown that the color and emissive properties of crystalline Pt(II) diimine salts are highly dependent upon the chosen anion and solvent(s) for the precipitation/recrystallization and differences in the extent of π—π and/or Pt—Pt interactions are usually presumed to be the reason of these phenomena. However, practicable sensory devices for VOCs are still sparse, largely due to ineffective coupling of vapor inhalation (trigger) to emission change (response). Technologically, a “switch-on” sensor is more desirable than a “switch-off” one; that is, there is higher practical value for a solid sensory material that can provide a positive luminescent response for the appearance of vapors and gases.
U.S. Pat. No. 5,766,952 by K. R. Mann et al describes a vapochromic double-complex salt of platinum(II) which changes its color, absorption or emission spectra upon exposure to VOCs, and can therefore be used in sensor devices for environmental evaluation. These Pt(II) complexes are chemically represented by the formulae [Pt(CN—C6H4-alkyl group)4][Pt(CN)4] and [Pt(diimine)(CN—C6H4-alkyl group)2][Pt(CN)4]. The deviations of Pt—Pt contacts induced by vapor inhalation are believed to account for the changes in the electronic spectra. However, such complexes are “switch-off” sensors showing negative luminescent response in the presence of CH2Cl2.
Very recently, a report on detection of volatile organic compound vapors by using a sol-gel material doped with a vapochromic complex of formula [Au(PPh2C(CSSAuC6H5)PPh2Me][ClO4] appeared on Appl. Phys. Lett. 2000, 77, 2274. However, the sensing mechanism relies on vapor-induced changes in the refractive index of the film, which may give rise to false signals and therefore may not be specific.